Giz Explains: How Sunscreen Works (And Why You're Wrong About It)

Your skin shouldn’t look like a package of pork cracklins after spending the day outdoors; that’s why we invented sunscreen. However, there’s a right way and a wrong way to slather on your protection — screw it up and you could get burned.

It’s not the visible light from the sun that causes your skin to crisp but rather the invisible ultraviolet waves that give sunlight its warmth. As assistant professor of dermatology and director of photomedicine at Tufts University School of Medicine, Jeffrey M. Sobell, explains:

A sunburn — manifested by cutaneous redness, swelling and pain — is an acute toxic reaction caused by exposure to the sun’s ultraviolet radiation. Although the precise mechanism by which a sunburn occurs has not been clearly identified, complex chemical reactions and pathways take place that most likely result in the clinical symptoms.

The energy from ultraviolet radiation can damage molecules in the skin, most importantly DNA. One consequence of this is the synthesis of different proteins and enzymes. The effects of these proteins, notably prostaglandins and cytokines, lead to dilation of the cutaneous blood vessels and recruitment of inflammatory cells. This, in turn, produces a sunburn’s characteristic redness, swelling and pain. Once the signal of excessive radiation exposure is initiated, it generally takes four to six hours for these proteins to generate. Sunburn symptoms thus don’t appear until well after exposure. (DNA damage can also result in the destruction of the involved skin cell. This is one of the reasons why skin peels after a bad sunburn.)

Ultraviolet radiation arrives most often in three distinct wavelengths, designated UVA, UVB and UVC. UVA has the longest wavelength (400nm-320nm) and therefore penetrates deeper into your skin than the others, causing age spots, wrinkling and other signs of premature ageing. UVA is also unimpeded by both the ozone layer and glass. UVB, with a wavelength of 320nm-290nm, is partially blocked by the ozone layer and cannot travel through glass. This wavelength is the primary cause of sunburns and the targeted range for protection, though broad-spectrum sunscreens will inhibit both UVA and UVB. UV-C has a wavelength of 290nm-100nm, although it is completely blocked by the ozone layer and is only an issue if you use tanning beds.

To block these harmful solar emissions, sunscreens employ a mix of organic and inorganic ingredients that either reflect, scatter or absorb the radiation and dissipate it as heat. Titanium dioxide and zinc oxide are two of the most commonly used inorganic materials used to physically block UV rays from reaching the skin. Previously, these ingredients would appear white (see below), however modern micronising techniques break down the particles so as to be invisible without diminishing their functionality.

These inorganic compounds are supplemented with UVB absorbing organic chemicals, such as cinnamates, octyl methoxycinnamate, para-aminobenzoic acid and Benzophenones that convert the incoming energy into harmless waste heat.

Together, these compounds provide a level of protection measured in factors. That is, if your baseline, unprotected time limit for sun exposure is say 30 minutes before burning, an SPF (sun-protection factor) 30 sunscreen would allow you to remain outside for 30 times that duration, or 15 hours. If your baseline is 15 minutes, an SPF would only hypothetically provide 7.5 hours of protection.

Of course, these products only work if you actually put it on and keep it on. Unfortunately, many folks fail to realise that fact and are putting themselves at long term risk for skin cancer and freckles by not doing so. These are a few of the most common sunscreen misconceptions:

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1. I’m naturally tan, so I don’t need sunscreen.

Hahaha, no. The Cancer Council reports that skin cancers account for around 80 per cent of all newly diagnosed cancers every year. Roughly 11,500 of them will be cases of invasive melanoma, skin’s deadliest cancer, and it will kill an estimated 1500 people. It’s not just the pastiest of us that are at risk; all skin burns eventually. Naturally darker skin simply offers a higher baseline protection factor, not an unstoppable UV barrier.

Instead, slather on a broad-spectrum sunscreen with an SPF of at least 30, water resistant if you have it. The SPF 30 will block 96 per cent of the harmful incoming rays, and the water-resistant properties will prevent it from washing away as you sweat.

“To cover your whole body, you would have to fill a shot glass,” dermatologist Dr Jennifer Stein of New York University Langone Medical Center said. It’s not as though you need to apply it to every crack and crevice, but be sure to get the tips of the ears and use lip balm with a sufficient SPF.

2. I got my face, neck, legs and forearms this morning, I’m good for the day.

Sunscreen does not last all day. “The general principle is to reapply every two to four hours,” dermatologist James Spencer, MD, of St Petersburg, Florida, told WebMD. “Sunscreen does go away with time.”

In addition to touching up your exposed bits every couple of hours, you should try to cover up as much as possible with long sleeves and broad-brim hats and avoid being under the sun when it’s at its midday peak between 10am and 2pm.

3. It’s cloudy out, I’m fine.

The weather makes little difference with overcast conditions blocking as little as 20 per cent of the inbound radiation. What’s more, geographic features like sand and water can all reflect as much as 70 per cent of the sunlight that hits them, and higher altitudes actually increase UV exposure (yes, because you’re closer to the sun). As such, you shouldn’t rely on the local forecast to protect you.

4. Anything higher than SPF 30 is a waste.

That’s partially true, but don’t count out the higher stuff entirely. SPF 90 only provides a 3 per cent boost in protection over SPF 30. “You get 99 per cent sunburn protection with SPF 90, versus 96 per cent with SPF 30,” says Howard Sobel, MD, of Lenox Hill Hospital in New York City.

For most people, that extra bit of protection may not be all that necesssary. However for exceptionally fair skinned folk that extra three per cent over a lifetime really adds up, and could determine whether or not you come home looking like Zoidberg.

"higher altitudes actually increase UV exposure (yes, because you’re closer to the sun)" Really? I mean seriously you can't be serious. The miniscule difference in distance to the sun is not the reason. At higher altitude the atmosphere is thinner which means it doesn't block as much UV radiation as it does at low altitude. Distance from the equator and time of year also make a difference with less penetrative due to the effectively thicker atmosphere with oblique angles.

"but rather the invisible ultraviolet waves that give sunlight its warmth." I think thermal radiation is more in the visible and infrared wavelengths, isn't it? That's why a cool, overcast day can still burn without feeling warm.

Not even remotely. You can hold an infrared laser to your arm for an hour and not have an effect, similarly for a visible light laser pointer.
The 'heat' that you feel is the energy from the lower wavelength bands, of which the clouds only block 30%.

That's a common misconception. The wavelength of emissions from a hot object actually depends on the temperature. Most people probably just associate infrared with thermal radiation because that's how we measure it most of the time on Earth, when looking for humans and animals in dark, cold environments. The sun however, emits light in infrared, visible light, and UV, because it is hot, and things that can absorb those wavelengths will also heat up.